This invention relates to an ultrasonic probe for use in an ultrasonic diagnostic apparatus and the like.
In application of an ultrasonic probe to medical uses, it is desirable that the piezoelectric member functioning as an electro-ultrasonic converter is composed of a material which not only has a large electro-ultrasonic conversion efficiency, in other words, a large electro-mechanical coupling coefficient, but is also soft and small in acoustic impedance. However, the inorganic materials with a large electro-mechanical coupling coefficient such as lead zirconate titanate (PZT) type ceramics are hard and high in acoustic impedance and have poor adaptability to the human body. On the other hand, the soft materials such as polymer have no piezoelectric property, or if they have piezoelectric property, they are small in electro-mechanical coupling coefficient. In order to overcome such dilemma, many attempts have been made recently for combining an inorganic material such as PZT ceramic and polymer to produce a composite material which possesses advantages of said both types of material. A pioneer study of this line has been done by Newnham et al of U.S., and the utility of such composite material is described in, for instance, Material Research Bulletin, Vol. 13, pp. 525-536. Various methods have been deviced for compounding of PZT ceramic and polymer. For instance, usefulness of a composite honeycomb structure illustrated in FIG. 1 of the accompanying drawings is disclosed in Material Research Bulletin, Vol. 15, pp. 1371-1379. The composite piezoelectric material 11 shown there comprises a honeycomb structure of PZT ceramic having a plurality of cavities 12 filled with polymer, and it is stated that the performance index of this composite piezoelectric material 11 in use for a hydrophone far excels that of PZT ceramic itself.
There have been also proposed the composite piezoelectric materials of a structure in which a plurality of columnar piezoelectric bodies are embedded in a plate of polymer having a uniform thickness. This type of composite piezoelectric materials have the same effect as the above-said type of composite material (shown in FIG. 1).
The present invention is an adaptation of a composite piezoelectric material such as mentioned above to an ultrasonic probe which receives and transmits ultrasonic waves. A prominent advantage of such composite piezoelectric material is that this material can be reduced in accoustic impedance and dielectric constant without lowering the piezoelectric strain constant (d constant) which has direct relation to the ultrasonic wave generation efficiency. This, however, is based on the supposition that (I) the piezoelectric ceramic existing dispersedly in the composite material is free of any lateral mechanical coupling, and (II) the whole composite piezoelectric material can be displaced uniformly in the thicknesswise direction.
In use of a composite piezoelectric material for an ultrasonic probe, the condition of (II) is especially important for the proper formation of beams. The two conditions, however, conflict with each other: for meeting the condition (I), the polymer must be soft, but the soft polymer can not fulfill the condition (II), that is, the polymer is required to be hard (to certain extent) for allowing a uniform displacement of the whole composite piezoelectric material in its thicknesswise direction.